One major objective of this work is to understand the role of uracil-DNA glycosylase in the metabolism of DNA. By isolating an E. coli mutant deficient in uracil-DNA glycosylase (ung), and combining this with mutants deficient in dUTPase (dut), we have been able to grow E. coli containing uracil in its DNA. The properties of the uracil-containing DNA will be investigated in vivo. By using bacteriophage unable to induce dUTPase activities, we are also able to synthesize phage with uracil-containing DNA. It is possible to vary the percent replacement of thymine by uracil in phage DNA over a range from 0% to about 30%. These phage will be used to study the effect of uracil in the DNA on expression of that DNA. Another objective of this work is to investigate the repair of DNA containing thymine dimers. Several kinds of preliminary experiments have suggested that endonucleases specific for AP sites in DNA play a role in repair of DNA containing thymine dimers. Thus, thymine-dimer initiated repair and uracil initiated repair may share a common repair pathway in vivo.